Roland Jäger

Cavity solitons as pixels in semiconductor microcavities

Cavity solitons are localized intensity peaks that can form in a homogeneous background of radiation. They are generated by shining laser pulses into optical cavities that contain a nonlinear medium driven by a coherent field (holding beam). The ability to switch cavity solitons on and off and to control their location and motion by applying laser pulses makes them interesting as potential ‘pixels’ for reconfigurable arrays or all-optical processing units. Theoretical work on cavity solitons has stimulated a variety of experiments in macroscopic cavities and in systems with optical feedback. But for practical devices, it is desirable to generate cavity solitons in semiconductor structures, which would allow fast response and miniaturization. The existence of cavity solitons in semiconductor microcavities has been predicted theoretically, and precursors of cavity solitons have been observed, but clear experimental realization has been hindered by boundary-dependence of the resulting optical patterns—cavity solitons should be self-confined. Here we demonstrate the generation of cavity solitons in vertical cavity semiconductor microresonators that are electrically pumped above transparency but slightly below lasing threshold. We show that the generated optical spots can be written, erased and manipulated as objects independent of each other and of the boundary. Numerical simulations allow for a clearer interpretation of experimental results.

Large-area single-mode VCSELs and the self-aligned surface relief

The effect of mode-profile specific etching of the top layer in selectively oxidized vertical-cavity surface-emitting laser (VCSEL) structures at 850-nm emission wavelength is examined. For high reproducibility, a self-aligned etching technique is demonstrated which aligns surface etch and oxide aperture by only one additional photoresist step. By optimizing layer structure and etch spot size, completely single-mode devices with aperture diameters up to 16 /spl mu/m are obtained. Maximum single-fundamental-mode output power of 3.4 mW at room temperature and over 1 mW at 0/spl deg/C is obtained with a maximum far-field angle of 5.5/spl deg/. Using parameters for etch spot height and diameter, Gaussian beam spot size and phase curvature, the measured diffracted far-field distribution is fitted well over a 20-dB intensity range. The chosen fit parameters therefore enable one to estimate the amount of phase curvature within the VCSEL for different operation currents, which cannot be obtained with available measurement methods.

Improved output performance of high-power VCSELs

The intention of this paper is to report on state-of-the-art high-power vertical-cavity surface-emitting laser diodes (VCSELs), single devices as well as two-dimensional (2-D) arrays. Both approaches are studied in terms of electrooptical characteristics, beam performance, and scaling behavior. The maximum continuous wave (CW) output power at room temperature of large-area bottom-emitting devices with active diameters up to 320 /spl mu/m is as high as 0.89 W, which is to our knowledge the highest value reported for a single device. Measurements under pulsed conditions show more than 10-W optical peak output power. Also, the CW performance of 2-D arrays has been increased from 0.56 W for 23 elements to 1.55 W for 19 elements due to significantly improved heat sinking. The extracted power densities spatially averaged over the area close to the honeycomb-like array arrangement raised from 0.33 kW/cm/sup 2/ to 1.25 kW/cm/sup 2/. Lifetime measurements have proven acceptable reliability for over 10000 h at a degradation rate of less than 1% per 1000 h. The emission wavelength of bottom-emitting devices is restricted to about 900 nm or higher due to fundamental absorption in the GaAs substrate. Windowing of the substrate has been studied to allow for shorter wavelength emission.

Multistage bipolar cascade vertical-cavity surface-emitting lasers: theory and experiment

We present an overview over our research on bipolar cascade vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm wavelength, including the scaling properties and the influence of design variations on laser performance as well as strategies for GaAs Esaki junction optimization. We experimentally demonstrate high-performance two- and three-stage devices, the latter of which with 130% differential quantum efficiency. The derived analytical expressions for the scaling behavior are confirmed by measurement data and show a significant improvement in the steady-state as well as the dynamic performance with respect to active region stacking. From the investigated design variations, the influence of oxide apertures and active region spacing on laser performance is clearly extracted and reveals that current spreading is present in the cavity and can lead to severe limitations in optical performance. The GaAs tunnel diodes have been optimized with respect to the donor concentration and the additional incorporation of an In/sub 0.1/Ga/sub 0.9/As layer on the n/sup ++/-side, leading to a zero-bias specific resistance of about 1/spl middot/10/sup -4/ /spl Omega/cm/sup 2/.

High volume production of single-mode VCSELs

Up to now applications for singlemode VCSELs were in low volume and high prized applications like tunable diode laser absorption spectroscopy (TDLAS, [1,2]) or optical interferometers. Typical volumes for these applications are in the range of thousands of pcs per year, with pricing levels of several 100 USD/pcs. New applications for singlemode VCSELs in consumer markets require manufacturing in very high volumes and at very low cost. Examples are laser-based optical mouse sensors, optical encoders, and rubidium atomic clocks for GPS systems [3,4]. U-L-M photonics presents manufacturing aspects, device performance and reliability data for these devices. The first part of the paper is dealing with high volume manufacturing of 850 nm singlemode VCSEL chips with very high efficiency and low operation current. Special processing technologies have been developed to achieve yields on 3 inch wafers of more than 90%. Wafer qualification procedures are discussed as well. The second part of the paper covers high volume packaging in TO and SMT type packages where very high packaging yields must be achieved. In the last part of the paper reliability issues are discussed, focused on the very high susceptibility of these devices to electrostatic discharge.

Volume production of polarization controlled single-mode VCSELs

Over the past 3 years laser based tracking systems for optical PC mice have outnumbered the traditional VCSEL market datacom by far. Whereas VCSEL for datacom in the 850 nm regime emit in multipe transverse modes, all laser based tracking systems demand for single-mode operation which require advanced manufacturing technology. Next generation tracking systems even require single-polarization characteristics in order to avoid unwanted movement of the pointer due to polarization flips. High volume manufacturing and optimized production methods are crucial for achieving the addressed technical and commercial targets of this consumer market. The resulting ideal laser source which emits single-mode and single-polarization at low cost is also a promising platform for further applications like tuneable diode laser absorption spectroscopy (TDLAS) or miniature atomic clocks when adapted to the according wavelengths.

Comparison of approaches to 850-nm 2D VCSEL arrays

There is a wide variety of reasons why future high-performance datacom links are believed to rely on two-dimensional VCSEL arrays suitable for direct flip-chip hybridization. Some typical are as follows: highest interconnect density, high-frequency operation, self alignment for precise mounting, productivity at high number of channels per chip. In this paper the latest approaches to flip-chip VCSELs are presented. In particular we will asses the properties of transparent substrate VCSEL arrays which are soldered light-emitting side up as well as VCSEL arrays which are soldered light-emitting side down, e.g., onto a CMOS driver chip. The VCSEL arrays are designed for bottom- or top-emission at 850 nm emission wavelength and modulation speeds up to 10 Gbps per channel.

High performance selectively oxidized VCSELs and arrays for parallel high-speed optical interconnects

High-bandwidth single-mode selectively oxidized vertical-cavity surface-emitting laser (VCSEL) arrays operate at 980 nm or 850 nm emission wavelength for substrate or epitaxial side emission. Coplanar feeding lines and polyimide passivation are used to reduce electrical parasitics in top-emitting GaAs and bottom-emitting InGaAs VCSELs. To enhance fundamental single-mode emission for larger devices of reduced series resistance a surface relief transverse mode filter is employed. Fabricated VCSELs are applied in various interconnect schemes. InGaAs quantum-well based VCSELs at 935 nm emission wavelength are investigated for use in perfluorinated graded-index plastic-optical fiber (GI-POF) links. We obtain a 7 Gb/s pseudo random bit sequence (PRBS) nonreturn-to-zero (NRZ) data transmission over 80 m long 155 /spl mu/m diameter GI-POF. We investigate data transmission over standard 1300 nm, 9 /spl mu/m core diameter single-mode fiber with selectively oxidized single-mode GaAs and InGaAs VCSELs. We achieve biased 3 Gb/s and bias-free 1 Gb/s pseudo-random data transmission over 4.3 km at 830 nm emission wavelength where a simple fiber mode filter is used to suppress intermodal dispersion caused by the second order fiber mode. For the first time, we demonstrate 12.5 Gb/s data rate transmission of PRBS signals over 100 m graded-index multimode fiber or 1 km single-mode fiber using high performance single-mode GaAs VCSELs of 12.3 GHz modulation bandwidth emitting at /spl lambda/=850 nm.

Integrated photodiodes complement the VCSEL platform

Many VCSEL based applications require optical feedback of the emitted light. E.g. light output monitor functions in transceivers are used to compensate for thermally induced power variation, power degradation, or even breakdown of pixels if logic for redundancy is available. In this case integrated photodiodes offer less complex assembly compared to widely used hybrid solutions, e.g. known in LC-TOSA assemblies. Especially for chip-on-board (COB) assembly and array configurations, integrated monitor diodes offer a simple and compact power monitoring possibility. For 850 nm VCSELs the integrated photodiodes can be placed between substrate and bottom-DBR, on top of the top-DBR, or inbetween the layer sequence of one DBR. Integrated intra-cavity photodiodes offer superior characteristics in terms of reduced sensitivity for spontaneously emitted light [1] and thus are very well suited for power monitoring or even endof- life (EOL) detection. We present an advanced device design for an intra-cavity photodiode and according performance data in comparison with competing approaches.

VCSEL arrays for high aggregate bandwidth of up to 1.34 Tbps

Even though the lane speed of VCSEL based AOC and transceivers has reached 25 Gbps and beyond [1-7], parallel optics are getting even more important in order to meet the increasing demand for aggregate bandwidths in upcoming applications, among others, 100 Gigabit Ethernet, Infiniband EDR, or EOM (embedded optical modules). As 100 Gbps can be achieved by, e.g., 4 times 25 Gbps using standard QSFP form factor, different approaches are using large scale 2D VCSEL arrays operating at lower lane speeds. Early work on 2D VCSEL based transceivers has already been presented beginning of this century [8] and recent work also addressed the potential of this technology [9,10]. In 2013, Compass EOS has introduced a 1.34 Tbps core router solution [11,12,13] that incorporates 2D VCSEL arrays of 14x12 emitters designed and manufactured by Philips U-L-M Photonics. The VCSEL array is mounted face down onto a CMOS ASIC, directly on top of the analog area. The emission wavelength of 1000 nm allows for substrate side emission and thus for flip-chip mounting as well as the possibility of integrating 2D microlens arrays onto the stack of CMOS and VCSEL array. After briefly introducing the router with regard to the incorporated VCSEL technology we discuss the design and performance of the VCSEL array. Finally, the assembly solution for this most compact and dense transceiver solution is presented.